I'm running for president. I am asking you to join me today as part of an unprecedented and historic grassroots campaign that will begin with at least 1 million people from across the country. Say you're in: https://t.co/KOTx0WZqRfpic.twitter.com/T1TLH0rm26

The 77-year-old self-described democratic socialist challenged Hillary Clinton in the Democratic presidential primary in 2016 and said on Tuesday that he planned to again seek the nomination.

Sanders has reshaped Democratic politics and earned a loyal following with his passionate defense of liberal proposals including free college tuition and single-payer health care. But he will face off against several other Democratic candidates who also want to appeal to the party's base.

Still, Sanders' name recognition, fundraising prowess and passion for liberal policies make him a top-tier 2020 presidential contender.